Deterioration due to mould attack occurs extensively particularly in so-called half-dry food articles and feeds such as bread, cheeses, smoked fish, dry sausages, air dried fruits, and air dried fish, especially when they are stored in packaged condition. Industrial food processing plants incur, through mould attack, losses which are difficult to assess, both in the form of returned, deteriorated products and of impaired sales owing to the products' inferior keeping quality. The consumers, too, are caused direct losses by such deterioration, but in addition they also run health risks because of the toxins formed by moulds, which may already be produced before the growth of mould is visually observable. One such dangerous toxin to which great attention has been paid in recent years is aflatoxin. Heretofore, attempts have been made to prevent mould formation by using packaging materials permeable to moisture, by intensifying the plant hygiene and thus reducing the amount of mould infection, and by using chemical compounds which destroy moulds or inhibit their growth. The use of moisture-permeable packaging materials has remained rather restricted, because it is desired to prevent excessive drying of the foods during storage. Intensifying the plant hygiene has successfully lowered the frequency of mould formation to a significant degree. However, it is impossible in practice to solve the problem completely by this approach, because, for instance, even by filtering the intake air and by ultraviolet treatment it has not been possible to reduce to a sufficiently low level the mould infection by which bread is contaminated while cooling. With the present modes of application, chemical preservatives cannot be added at high enough concentrations to ensure adequate inhibition of mould formation.
Factors limiting the concentrations are: the flavour of the preserving agents, their inhibitory effect on fermentation (e.g. on addition to yeast dough), the physiologically harmful effects of high concentrations, and also legal restrictions.
Besides mould formation, a considerable part of other deterioration of foods and feeds is a result of growth and metabolism of aerobic microorganisms, which are deposited on the surface of food or feed through post-processing contaminations such as from air, from the hands of an operator, from equipment and utensils etc. Typical examples are the formation of slime on the surface of slaughtered animal carcasses cold storage, or the growth of bacterial colonies on sliced sausage. In these cases, too, the limitations of the heretofore available methods are in principle the same as mentioned above in regard to the prevention of mould formation.
Since the post-processing microbial contamination in most cases remains on the surface of the food or feed, and the moulds and other aerobic microorganisms, because of their oxygen requirements, generally can multiply only on the superficial layers of the food or feed, the measures aimed at the fighting of moulds and other aerobic microorganisms may usually be concentrated merely on the superficial layer, and the desired preventive effect can thereby be achieved. The procedures applied heretofore for the purpose of applying chemical or equivalent preventives of microorganisms into the superficial layer of the food have been: to dip the food into a solution of chemical preservative, to spray such a solution onto the surface of the food, and to impregnate the packaging material with a chemical preservative. When the first-mentioned method is used, the surface of the food is wetted by water or another solvent, and this is undesirable in many instances. Wetting of the surface is also incurred when the solution is sprayed. Furthermore, the sprayed fluid droplets proceed along straight paths, and therefore all points of the surface are not treated with certainty even though spraying be performed from more than one direction. This is particularly true as regards the depressions and pores in the surface. When the chemical preservative is applied by mediation of the packaging material, those instances are rare in which a good enough contact is obtained between the packaging material and the food to ensure that the concentration of the preserving agent will be high enough at all points of the surface.
Attempts to apply aerosols for surface disinfection in general have so far been unsuccessful. This holds true also for smoking procedures. The preserving agents of wood smoke consist mainly of phenolic compounds and formaldehyde. The concentrations of these compounds on the surface of foods at the end of the smoking process remain usually so small that their inhibiting effect on the microbial growth is only partial. In addition, the concentrations in the smoke of individual chemical compounds effective against microorganisms are not constant, and the smoking processes are not controlled with a view to these effects but rather to the main objectives of the smoking, i.e. the flavour of smoke and the drying of foods. As a result, smoked foods are susceptible to mould formation and to the growth of certain bacteria and yeasts tolerant to low water activites, unless the surface is kept sufficiently dry. In fact, certain types of smoked sausages are ripened with the aid of moulds originating either from natural contamination or added as pure cultures. Likewise, toxin-producing staphylococci are able to multiply on the surface of smoked fish, and this food item is known to cause frequently staphylococcal food poisonings.
The application of aerosols using prior art methods in the treatment of growing vegetables or fruits or other living tissues leads in several cases to thoroughly wetted surfaces, before the treatment is completed. In other cases, where the surface is not totally covered by the substance sprayed, the substance effective against microorganisms can during the subsequent growing period either diffuse or is actively transported by the living tissue also to the uncovered parts of the surface. Such treatments are not usually possible for most other foods or feeds.